Circular wave guides are used in telecommunications; they have an internal conductive surface in the form of a cylinder of revolution through which electromagnetic waves propagate at various frequencies, lying conventionally between 30 and 100 Ghz.
Signals can thus be transmitted over long distances with a very wide pass-band. Unfortunately, these signals are progressively distorted because the propagation velocity of their various components increases with their frequency. This propagation velocity is indeed the group velocity of the waves in the guide, it being known that this velocity is different from the phase velocity and that it increases with the frequency. To obtain signals without distortion, it is therefore necessary to place delay equalizers in the signal transmission circuit. Such an equalizer should delay the various components of the signals, the delay increasing with frequency to compensate the advance acquired by the high-frequency components in the guide.
Delay equalizers are known for rectangular wave guides. These equalizers using a set of guides connected together and forming a conventional device called a "hybrid T" and constituted by four arms: an input arm, an output arm perpendicular to the input arm and two lateral arms alined perpendicular to the input arm and to the output arm. There is a difference of a quarter of a wavelength between the lengths of these lateral arms and each of them is terminated with a "progressive reflector", i.e. at a guide having a decreasing cross-section. The function of this progressive reflector is to reflect the waves which penetrate therein after they have travelled along a path which increases with their frequency. It is known that in these conditions the waves arriving through the input arm are transmitted to the output arm with a delay which increases with their frequency because the higher frequency waves have travelled along a longer path in the progressive reflectors.
Such a delay equalizer for a rectangular wave guide can be used with circular wave guides only in conjunction with transition elements between the circular wave guides and rectangular wave guides. Such transition elements make the telecommunications devices more complex and more expensive.
It is also known to produce delay equalizers for circular wave guides, which transpose the frequency of the waves propagated in the guides so as to obtain signals at much lower frequencies (medium frequency) which can be handled by conventional electronic circuits. These electronic circuits are designed to delay the various components of the signals by amounts which are greater for components transmitted along the transmission line at higher frequencies. Such circuits are complex and expensive.
Preferred embodiments of the present invention provide cirular wave guide delay equalizers which are simple to manufacture.